An aqueous dispersion of urethane fine particles, such as polyurethane polyurea particles, has been prepared mostly by dispersing an isocyanate-terminated isocyanate prepolymer in water and adding a polyamine, etc. to the dispersion to increase the molecular chain length as described in JP-A-48-51949 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-62-67003, JP-A-62-149333, and JP-B-63-58610 (the term "JP-B" as used herein means an "examined Japanese patent publication").
However, it has been difficult to obtain a film having satisfactory properties from the urethane fine particles obtained by these conventional techniques due to the poor solvent resistance of the film formed.
In order to improve solvent resistance it has been proposed to use a combination of an isocyanate prepolymer having self-dispersibility and a polyisocyanate having a functionality of 2.1 to 4 as disclosed in U.S. Pat. No. 4,507,431 and JP-A-3-128912.
Although the above process is advantageous for obtaining finely dispersed crosslinked polymer particles capable of forming a solvent-resistant film, it encountered difficulty in preparing finely dispersed particles having encapsulated therein a core substance, such as a pigment, in a stable manner because the isocyanate prepolymer or the 2.1 to 4-functional polyisocyanate often tends to react with the water content present in the core substance to cause such an unfavorable phenomenon as viscosity increase and relation.